Liz Goldsmith,
tia Fiber Optics lan Section
In his Endface article that appeared in the February 1998 issue (see "The impending fiber-bandwidth shortage," page 96), Scott Stevens makes several misleading statements. The members of the tia Fiber Optics lan Section (fols)--3M, amp, Allied Telesyn Interna- tional, Belden Wire & Cable, Berk-Tek, Corning, Lucent Technologies, Siecor Corp., Spectran, Sumitomo Electric Lightwave, and Transition Networks--would like to address the central issues raised by the article, namely, how much bandwidth do users need to meet current and future requirements, and is there an advantage to purchasing much higher-bandwidth fiber than recommended in the standards?
All fiber applications today--and those that will be deployed in the immediate future--are specified with one or more of three standard media types: 62.5- and 50-micron multimode fiber, and singlemode fiber. These fiber types offer different standard specifications to accommodate different application and distance requirements. A higher-bandwidth medium will not make a system run faster because system speed is a function of electronic components. Also, any longer distances that higher bandwidth might enable are appropriate for limited applications and must be evaluated accordingly.
The performance parameters of standard fiber types have not changed over the past decade because there has been no requirement for them to change. Unlike copper alternatives, standard fiber types offer inherent performance that exceeds the bandwidth demands of most applications. Optical fiber is the only transmission medium that provides users with a clear migration path from 10-megabit-per-second Ethernet to faster protocols, including Gigabit Ethernet. A multimode fiber-based system purchased today will meet current needs and can be used without modification for many gigabit-speed links.
Recognizing that standard fiber bandwidths have served the industry well, the fols also realizes that the marketplace is becoming increasingly complex and that some emerging applications demand higher bandwidth. Technologically, several options exist for extending multimode fiber`s capability. Multilevel coding increases a medium`s transmission capacity using less bandwidth and is popular in copper-based local area networks. Wavelength-division multiplexing, which provides multiple transmission channels over one fiber by using different wavelengths (colors) of light, is another option. For currently deployed applications, bandwidth increases in the first operating window do not offer significant speed or distance enhancements.
A promising option
In addition, since 850-nanometer devices cost less than 1300-nm ones, standard multimode fibers are compatible with current trends in source technology. Vertical-cavity surface-emitting lasers, which offer laser-like performance with cost structures similar to light-emitting diodes, represent a promising option likely to offer both high performance and low cost. Putting higher second-window bandwidth to effective use requires 1300-nm laser-based devices to achieve the bit rates that might benefit from the higher bandwidth. Thus, while installing fiber with higher bandwidth at the 1300-nm window may seem like a prudent migration path to future protocols, most emerging applications for multimode fiber will be in the 850-nm window.
The way in which the ieee 802.3z Committee is developing the Gigabit Ethernet specification demonstrates that the communications industry is commited to ensuring the integrity of the installed base of fiber when developing standards. It is also important to look at all the work undertaken by the ieee 802.3z Committee. Discussions that pertain to the standard`s capability are not restricted to fiber as a limiting factor. The Physical Media Dependent Subcommittee is looking closely at active network electronics in order to fully understand issues such as random and deterministic jitter in fiber receivers.
The key to determining the appropriate bandwidth and fiber type for a particular link distance is matching the capability of the fiber with that of the current and anticipated device technologies. Specify appropriate bandwidth performances and look for manufacturers with proven records of manufacturing high-quality products tested in accordance with industry- approved methods and who stand behind their products` long-term performance.